Axial Piston Machine having a Retraction Plate, and Method for the Production Thereof

ABSTRACT

An axial piston machine comprises a cylinder drum in which a multiplicity of pistons are arranged in a longitudinally displaceable manner. The pistons are supported, via sliding blocks, on a running surface which is inclined relative to the cylinder drum axis. The sliding blocks are held, at that side thereof which faces away from the running surface, by means of a retraction plate. The retraction plate is produced from processed sheet metal in a non-cutting punching and stamping process.

The invention relates to an axial piston machine having a retraction plate and to a method for the production of the retraction plate.

In an axial piston machine, pistons are arranged longitudinally displaceably in the axial direction in a rotatable cylinder drum. These are supported on a running surface inclined in relation to the cylinder drum axis. Where a hydraulic pump is concerned, pressure conditions in this case prevail during the suction stroke which relieve the sliding shoe completely of load and would therefore cause it to lift off from the running surface. In order to keep the sliding shoes in sliding bearing contact against the running surface in every operating state and thereby supplement the suction intake of pressure medium, a retraction plate is consequently provided. The retraction plate is arranged parallel to the running surface and engages with a plurality of recesses in each case around a sliding shoe. Such an arrangement and such a retraction plate are known from EP 1 561 031 B1. In order to keep the costs for manufacturing the retraction plate low, it is already proposed there to produce the retraction plate in a stamping/embossing operation. By means of the stamping/embossing operation, on the one hand, the passage orifices are stamped out of the basic body designed as a planar circular disk and then surround the sliding shoe. On the other hand, at the same time, a bearing surface for supporting the retraction plate is formed. After this forming step, a planar surface which subsequently transmits the retention force to the sliding shoe is provided. This subsequent reworking is carried out by cutting and may include additional heat treatment, thus signifying a considerable additional outlay in manufacturing terms.

The object on which the invention is based is to provide an axial piston machine having a retraction plate and also a method for the production of the retraction plate, said method being simplified as compared with the known retraction plates.

The axial piston machine according to the invention has a cylinder drum in which are arranged longitudinally displaceably a plurality of pistons. The pistons are supported via sliding shoes on a running surface inclined with respect to the cylinder drum axis, the sliding shoes being held, so as to bear against the running surface, by means of a retraction plate on a retention surface facing away from their sliding surface bearing against the running surface. According to the invention, the retraction plate is produced from refined sheet metal, without cutting, in a stamping/embossing operation. In the method according to the invention, when the retraction plate is produced, first a refined metal sheet with a surface finish-machined so as to bear against the sliding shoes is generated. This refined metal sheet is subsequently subjected to a stamping/embossing operation, with the aid of which the sliding shoe reception orifices are introduced. The method according to the invention and the axial piston machine according to the invention are, in particular, more cost-effective than the known methods, and also weight can be saved on the retraction plate. Using or generating a refined metal sheet before a forming process takes place has the advantage that remachining by cutting and/or thermal treatment may be dispensed with. The sliding properties are already determined by the refined metal sheet itself.

The sub claims relate to advantageous developments of the axial piston machine according to the invention and of the method according to the invention.

In particular, it is advantageous to form on the retraction plate a region which is in the form of the envelope of a cone frustum. This region is shaped such that, in the nonloaded state, the distance of the retraction plate at its outer circumference from the running surface is shorter than in the region of the retraction plate bearing. Higher forces upon the sliding shoe are thus generated in the region facing radially away from the cylinder drum axis. The tendency of the sliding shoe to lift off under the action of centrifugal forces on the outside is consequently counteracted. Such prestress is advantageous especially when the refined metal sheet has spring-elastic properties, so that, after mounting, tension is generated in the retraction plate, since the latter is supported, on the one hand, in the region of the retraction plate bearing and, on the other hand, in the outer region on the sliding shoes.

Furthermore, it is advantageous, in order to save weight, that an outer circumferential edge of the retraction plate runs in each case partially parallel to an outer region of a sliding shoe reception orifice in each case. Consequently, the retraction plate has no circular outer geometry and the material present between the individual sliding shoe reception orifices at the outer circumferential margin can be removed. The use of material can thus be diminished and therefore the weight of the retraction plate can also be reduced. The outer geometry shaped in this way is in this case preferably generated simultaneously with the forming operation in a stamping/embossing process without cutting.

The retraction plate bearing of the retraction plate is preferably designed in the form of a spherical cap and without any passage orifice. In particular, in this case, that side of the metal sheet which faces away from the side co-operating with the sliding shoes is also refined such that low frictional forces occur between the retraction plate bearing configured in the form of a spherical cap and the counterpiece of the axial piston machine. Such a retraction plate bearing in the form of a spherical cap has especially the advantage that automatic centering of the retraction plate takes place even when the angle of inclination of the running surface changes. Furthermore, advantageously, a corresponding depression may be arranged in the running surface itself, so that the retraction plate comes to bear there permanently and therefore a constant distance is ensured. Moreover, the supporting force generated on the retraction plate by means of the retraction plate bearing may be used at the same time for keeping the pivoting cradle, on which the running surface is formed, in position and, in particular, in its pivoting cradle bearing.

The diameter of the retraction plate bearing in the form of a spherical cap is in this case preferably selected such that this diameter is equal to the diameter of the sliding shoe reception orifices.

The stamping/embossing operation preferably takes place in a plurality of successive steps. In this case, in one step, after the refined metal sheet has been generated, first a molding corresponding to a circular disk is shaped so as to form the region which is in the form of a cone envelope. In this case, in cross section, the surface ultimately forming the bearing surface forms an angle of less than 180°.

The introduction of the retraction plate bearing in the form of a spherical cap is preferably carried out by means of an embossing operation and, in particular, may be carried out simultaneously with the generation of the geometry in the form of the envelope of a cone frustum. The center point of the geometry in the form of a spherical cap and the apex of the cone frustum lie in this case in the same side of the retraction plate.

The axial piston machine according to the invention and the corresponding method for the production of the retraction plate are explained in more detail by means of the following explanations and the accompanying drawings in which:

FIG. 1 shows a part section through an axial piston machine according to the invention to explain the co-operation of the individual parts of an axial piston machine;

FIG. 2 shows a section through a molding as an intermediate product in the generation of the retraction plate;

FIG. 3 shows a perspective illustration of a finished retraction plate; and

FIG. 4 shows a simplified illustration of the method to explain the method for the production of the retraction plate.

The axial piston machine 1 according to the invention is illustrated in a part section in FIG. 1. It has a housing 2 in which a cylinder drum 3 is arranged so as to be rotatably mounted. A plurality of cylinder bores 4, only one of which is illustrated for the sake of simplification, are introduced into the cylinder drum 3. The cylinder bores 4 are connected, via orifices 5 which issue on an end face of the cylinder drum 3, alternately to a high-pressure connection and a low-pressure connection of a working circuit when the cylinder drum 3 is rotated. For this purpose, the cylinder drum 3 is rotatable about its cylinder drum axis 6. The plurality of cylinder bores 4 are arranged, distributed uniformly, on a circumferential circle around the cylinder drum axis 6. On the side facing away from the orifice 5, the cylinder bores 4 are open toward the second end face of the cylinder drum 3. In each case a piston 7 is arranged longitudinally displaceably in the cylinder bores 4.

Each of the pistons 7 is connected to a sliding shoe 8 in an articulated manner. For this purpose, the sliding shoe 8 has a spherical head which engages into a corresponding recess of the piston 7. The recess or the piston 7 in this case surrounds the head to an extent such that tensile and compressive forces can be transmitted between the sliding shoe 8 and the piston 7. The sliding shoes 8 bear slidably, in each case with a sliding surface 9 formed on a foot region of the sliding shoe 8, against a running surface 10. The running surface 10 is formed on a pivoting cradle 11. The inclination of the pivoting cradle 11 in relation to the cylinder drum axis 6 can be adjusted by means of an actuating device 12. The adjustability of the inclination of the pivoting cradle 11 and therefore of the running surface 10 in relation to the cylinder drum axis 6 is already known per se. A detailed illustration is therefore dispensed with at this juncture. To absorb the compressive forces, the pivoting cradle 11 is supported in a pivoting cradle bearing 13. The pivoting cradle 11 is mounted rotatably in the latter.

During one revolution of the cylinder drum 3, the pistons 7 execute a lifting movement in their respective cylinder bore 4. Where a pump is concerned, in this case the cylinder drum 3 is driven and, during the delivery stroke, the volume enclosed in the cylinder bore 4 by the piston 7 is reduced and therefore a volume flow through the orifice 5 into the working circuit is generated. During further rotation of the cylinder drum 3, the volume enclosed by the cylinder bore 4 and the piston 7 has to be increased again. In order to hold the sliding shoe 8 with its sliding surface 9 against the running surface 10 during this suction stroke, a retraction plate 14 is provided. The retraction plate 14 has a number of sliding shoe reception orifices 15 which corresponds to the number of sliding shoes 8. The sliding shoe reception orifices 15 are in this case selected in terms of their diameter such that they can be placed over the head of the sliding shoe 8 onto the foot of the sliding shoe. The reflection plate 14 bears there with a bearing surface against a retention surface 16, formed at the foot, of the sliding shoe 8.

To generate the necessary retention force, the retraction plate 14 is supported by means of a retraction plate bearing 17 on a bearing bolt 18. The bearing bolt 18 is loaded by a helical spring 19 which acts upon the bearing bolt 18 in the direction of the pivoting cradle 11. The helical spring 19 and the bearing bolt 18 are inserted into a recess 20 which is formed as a blind bore in the cylinder drum 3. The recess 20 in this case extends in the direction of the cylinder drum axis 6.

As can be seen clearly in the exemplary embodiment illustrated, the retraction plate 14 is designed there in the form of a spherical cap in order to form the retraction plate bearing 17. The centerpoint of the spherical cap shape lies in this case on that side of the retraction plate 14 which faces away from the pivoting cradle 11. The bearing bolt 18 is configured spherically on its side facing away from the helical spring 19 and engages on the concave side of the retraction plate bearing 17 into the geometry which is in the form of a spherical cap.

As is also explained in detail below, the retraction plate 14 is manufactured without cutting from a refined metal sheet and therefore has an essentially constant material thickness. Changes in the material thickness arise merely as a result of manufacturing tolerances and due to the stretch of the material on account of the forming operation. In the region of the retraction plate bearing 17 in the form of a spherical cap, a convex side, directed toward the pivoting cradle 11, of the retraction plate bearing 17 is obtained. This convex side engages into a depression 21 of the pivoting cradle 11, so that the distance of the retraction plate 14 from the pivoting cradle 11 is always constant. By means of the retraction plate 14, therefore, the pivoting cradle 11 is also acted upon with a force in the direction of the pivoting cradle bearing 13 by the bearing bolt 18.

Details of the geometry of the retraction plate 14 are explained below with reference to FIG. 2 and FIG. 3. FIG. 2 illustrates first a cross section through a molding of a retraction plate 14, such molding being generated during production. The retraction plate bearing 17 in the form of a spherical cap can be seen in the middle of the retraction plate 14. A region A designed in the form of the envelope of a cone frustum adjoins radially toward the outer circumferential edge. This region A ensures that the distance of the bearing surface 22 from the running surface 10 decreases from the retraction plate bearing 17 in the form of a spherical cap toward the outer circumference of the retraction plate 14. Thus, with an increasing distance from the cylinder drum axis 6, the pressure force exerted upon the sliding shoes 8 by the retraction plate 14 via the bearing surface 22 becomes greater. The tendency of the sliding shoes 8 to lift off on their side further away from the cylinder drum axis 6 is consequently counteracted.

As can likewise be seen in FIG. 2, the centerpoint of the retraction plate bearing 17 in the form of a spherical cap and the apex of the cone frustum are formed on the same side of the retraction plate 14.

A perspective illustration of a finish-machined retraction plate 14 is shown in FIG. 3. In the production of the retraction plate 14, first, the geometry illustrated in FIG. 2 is generated from a basic shape which is initially in the form of a circular disk. This basic shape in the form of a circular disk is a refined metal sheet of constant material thickness t. The refinement in this case takes place particularly with a view to the surface quality of the bearing surface 22. That side of the metal sheet which faces away from this plays a minor part in terms of its quality. However, the region which is intended to produce the retraction plate bearing 17 in the form of a spherical cap may also be refined correspondingly. Refinement in this case takes case particularly with a view to the sliding properties.

After first forming by embossing to generate the retraction plate bearing 17 in the form of a spherical cap and the geometry of the region A which is in the form of the envelope of a cone frustum has been carried out, the plurality of sliding shoe reception orifices 15 are introduced in a subsequent stamping step. The sliding shoe reception orifices 15 are of circular design and are distributed uniformly over the circumference of the retraction plate 14. The diameter of the sliding shoe reception orifices 15 and the diameter of the retraction plate bearing 17 in the form of a spherical cap are preferably identical. The forming and stamping of the sliding shoe reception orifices 15 are carried out, starting from a basic body with a diameter D. In order to save material, in this case material can be removed in the marginal region by stamping. The remaining webs around the sliding shoe reception orifices 15 are in this case partially a constant width B. To make clear the partially constant width B of the webs, a segment 20 is illustrated. Over the largest part of this segment 20, the outer circumferential edge 19 of the retraction plate 14 is parallel to the outer region of the sliding shoe reception recess 15. Only toward the boundary of the segment 20 does the web width increase so as to reduce the notch effect at the transition to the adjacent segment.

A greatly simplified method sequence is illustrated once again in FIG. 4. First, in step S1, a basic body is produced from a refined metal sheet. This basic body, which is preferably a circular disk, is subsequently formed in a first forming step S2 into a molding such that a retraction plate bearing 17 in the form of a spherical cap and the region A in the form of the envelope of a cone frustum are generated. In step S3, the plurality of sliding shoe reception orifices are introduced into this molding by stamping. At the same time or in a subsequent step S4, the marginal region is adapted so that the partially parallel circumferential edge 19 is obtained. 

1. An axial piston machine comprising: a cylinder drum; a plurality of pistons arranged longitudinally displaceably in the cylinder drum; and a retraction plate produced from a refined metal sheet, without cutting, in a stamping/embossing operation, wherein the plurality of pistons are supported via sliding shoes on a running surface inclined with respect to a drum axis of the cylinder drum, and wherein the sliding shoes are held, so as to bear against the running surface by means of the retraction plate on a retention surface facing away from their sliding surface bearing against the running surface.
 2. The axial piston machine as claimed in claim 1, wherein the retraction plate has a region in the form of an envelope of a cone frustum which is shaped such that, in a nonloaded state, a distance of the retraction plate at its outer circumference from the running surface is shorter than in a region of a retraction plate bearing.
 3. The axial piston machine as claimed in claim 1, wherein a circumferential edge runs in each case partially parallel to an outer region of a sliding shoe reception orifice in each case.
 4. The axial piston machine as claimed in claim 1, wherein the retraction plate bearing includes a spherical cap in the retraction plate.
 5. The axial piston machine as claimed in claim 4, wherein a diameter of the spherical cap and a diameter of the sliding shoe reception orifices are equal.
 6. A method for the production of a retraction plate for an axial piston machine, comprising: generating a refined metal sheet with a bearing surface finish-machined so as to bear against the sliding shoes; stamping sliding shoe orifices into the refined metal sheet; and forming a retraction plate bearing in the refined metal sheet.
 7. The method as claimed in claim 6, wherein the metal sheet is formed such that, in a cross section through the retraction plate, the bearing surface forms an angle of less than 180°.
 8. The method as claimed in claim 7, wherein an outer region is removed by stamping, so that a circumferential edge of the retraction plate runs in each case partially parallel to an outer region of a sliding shoe reception orifice in each case.
 9. The method as claimed in claim 6, wherein the retraction plate bearing is introduced in the form of a spherical cap into the metal sheet by means of an embossing operation.
 10. The method as claimed in claim 6, further comprising: forming a molding from a planar metal sheet (i) by embossing the retraction plate bearing in the form of a spherical cap, and (ii) by generating a region in the form of an envelope of a cone frustum outside the retraction plate bearing. 